1. Field of the Invention
This invention relates to a transport refrigeration system capable of providing both heating and cooling and more particularly to a direct oil return circuit for returning oil from the bottom of the accumulator tank of such a system directly to the refrigerant compressor crankcase.
2. Description of the Prior Art
Reversible refrigeration systems (i.e., systems capable of operating in a heating mode or a cooling mode) generally include suction accumulator tank providing a general tranquil environment in which the refrigerant vapor and liquid can be separated. In the normal cooling mode, the refrigerant is vaporized in the evaporator and such accumulator tank essentially has no other function; however, in the heating mode of the particular reversible refrigeration system shown by U.S. Pat. No. 3,219,102, of common assignee, the accumulator tank provides the evaporator function in lieu of the normal condenser coils. Under such conditions, the accumulator tank functions as the evaporator in the system and has a substantial quantity of liquid refrigerant delivered to it. Also, during "off" periods there may be migration of the liquid refrigerant from the warmer evaporator to the accumulator tank such as when the temperature of the ambient air surrounding the tank and compressor (and thus the temperature of these components) is less than the temperature of the interior of a truck trailer housing the evaporator. By providing external heat to the accumulator tank, and a high-level discharge from the accumulator to the suction of the compressor, the liquid refrigerant therein is vaporized and discharged to the compressor, minimizing slugging damage that can occur when liquid refrigerant is ingested into the compressor cylinders. Also, an inherent characteristic of refrigeration systems is the tendency of the compressor to pump lubricating oil from the compressor discharge throughout the rest of the system. Without some special provision, such circulating oil in the above-described system would eventually become trapped in the bottom of the suction accumulator tank. Thus, such refrigeration systems generally utilize either one of two mothods for returning the lubricating oil from the accumulator tank to the compressor sump.
The first method comprises an aspirating device, such as an aspirating U-tube briefly described in commonly assigned U.S. Pat. No. 3,978,685, which includes a U-tube within the accumulator tank having a high-level refrigerant inlet (so as to receive the vapor in the tank) for discharging to the compressor inlet. The U-tube has an oil opening in the bottom, generally adjacent the bottom of the tank, wherein oil can be aspirated into the vapor stream flowing through the tube back to the compressor. Such oil return system has several deficiencies, including:
(1) The vapor flow restriction imposed by the oil aspiration system in the accumulator tank impairs the compressor capacity;
(2) Because of the vapor velocities within the compressor suction inlet chamber, an undesirably high proportion of the oil aspirated into the refrigerant vapor remains entrained in the refrigerant and recirculated therewith, thus penalizing the refrigerant mass flow and the heat transfer effectiveness of the various coils; and,
(3) The available oil supply in the compressor crankcase is diminished by the amount of oil in circulation and lodged in the accumulator.
Thus, a reduction in the amount of circulating oil and/or returning the oil to the compressor sump without the necessary vapor flow restriction of the aspiration system would improve the performance of the refrigeration system.
Another method of returning lubricating oil is through a separate oil-return line which runs directly from the bottom of the accumulator tank to the compressor crank case to drain oil directly to the sump. In this system, specific provisions are desirable to prevent crankcase ingestion of liquid refrigerant which may also be in the accumulator, such as a flow restriction to limit liquid refrigerant ingestion to allowable levels and a shut-off mechanism to prevent migration of liquid refrigerant to the compressor during the "off" cycle of the system.
In previous systems of this nature, the shut-off mechanisms have included solenoid actuated valves or liquid level actuated valves that prevented migration of the refrigerant to the sump during the "off" cycle. In tranport refrigeration systems, the use of liquid-level valves is unpredictable because of the vibration and inertial shocks to which they are subjected during over-the-road use. Further, such components add to the complexity and the cost of such a refrigeration system. Also, it is known to provide an external heat source for the oil return line from the accumulator tank to the compressor sump which has the dual effect of vaporizing most of the liquid refrigerant passing therethrough and which in turn causes a flow restriction in the return line, effectively metering the rate of the return flow.